The present invention relates to a method for preventing the deterioration of picture quality in a video processor and circuit thereof, and more particularly, to a method and circuit for preventing picture quality deterioration which is caused by aliasing created due to the change of a clock for a combfilter, because of the absence of a burst signal when processing a monochrome signal, in a digital combfilter which uses the clock responding to the burst signal.
Generally, a video signal processor for separating a video signal (for instance, an NTSC signal) into a luminance (Y) signal and a chrominance (C) signal uses a 1H combfilter Recently, a frame combfilter, which employs a frame memory in Y/C separation, has been in practical use.
The video signal processor is used for, among other things, processing a composite video signal. Generally, in order to help modulate the color components of the composite video signal, the processor has a burst-locked clock generator for generating a clock signal which is locked by a burst signal. For sampling and signal processing, the generated clock signal is provided to a digital signal processor. The burst-locked clock generator, normally is coupled to be responsive to the burst component of the composite video signal prior to the separation of the luminance and chrominance component.
The clock signal generated from the burst-locked clock generator for digital-combfilter-processing is generally processed such that the output of a 3.58 MHz (3.57945 MHz for NTSC) oscillator used in a video signal encoder or decoder is multiplied by four and expressed as 4fsc. The use of 4fsc facilitates signal processing, but can be varied.
FIG. 1 is a block diagram partially showing a video processor containing a conventional combfilter and clock a generator for the combfilter, each of which processes the Y signal and C signal by 1H.
In FIG. 1, before tile analog-to-digital (A/D) conversion of an input video signal by an A/D converter 102, a low-pass filter (LPF) 101 is employed as an anti-aliasing filter. To optimize resolution the bandwidth is designed so that the frequency (cut-off frequency) at a point at which gain is lowered below 3 dB is 7 MHz. In other words, LPF 101 greatly attenuates the signal above 7 MHz so as to remove high-band noise components and aliasing of a signal component is created below -3 dB.
Here, aliasing is a phenomenon in which frequency mixing between spacial high frequency components occurs when a video signal is subsampled. In order to prevent the aliasing, subsampling should be performed after low-pass-filtering. When the original spectrum of a video signal is sampled, repetitive spectra are created centering on sampling ratio fs. If the video signal is subsampled simply by two to one, spectrum folding of high frequency components is created. In order to properly subsample, according to the sampling frequency during the subsampling, baseband filtering (low-pass-filtering) should be performed in advance. By doing this, the spectrum has no aliasing.
When the attenuation characteristic of an anti-aliasing filter is ideal, interference due to the aliasing does not occur. However, in the case of an LC filter or Chebyshev filter used as the anti-aliasing filter at present, aliasing interference takes place above about 4 MHz.
However, in a moving picture, human eyes cannot perceive the phenomenon as noise. Even in a still picture, if the clock for a digital combfilter 103 is constant and does not change, the interference due to the aliasing is uniformly displayed on a screen, it is not offensive to the human eye.
A clock signal for the signal processing of digital combfilter 103 is output from a frequency multiplier 108. The clock signal is made by multiplying 3.58 MHz by four, which is locked by a voltage-controlled oscillator 107 of a phase-locked loop (PLL), while taking as a reference signal the burst signal of a composite video signal input CV. Here, the PLL corresponds to a comparator/control voltage generator 106 and voltage-controlled oscillator 107.
Meanwhile, if the input video signal is a monochrome signal (mono video signal) free from a color synchronizing signal (burst signal), since there is no reference signal of the PLL, the oscillating frequency of voltage-controlled oscillator 107, which generates 3.58 MHz, is varied.
The varied output of voltage-controlled oscillator 107 is multiplied by four in frequency multiplier 108, so as to be output as a sampling clock of A/D converter 102 and D/A converter 104 and a signal processing clock of digital combfilter 103. Thus, the sampling clock and signal processing clock of combfilter 103 are varied to be shown as frequency variation.
Due to this, if the input mono signal is a still picture, aliasing is created and is offensive to the human eye, thereby deteriorating picture quality.
A video signal processor system incorporating a burst locked clock generator responding to a separated color signal provided by a combfilter is disclosed in U.S. Pat. No. 5.132.784.
The combfilter disclosed in the above US patent passes an uncombfiltered video signal during a burst period in response to a burst gate signal, so as to improve the response time of the clock generator.
Though, during intervals other than the burst period, the system generates a phase-controlled clock signal using the separated color signal and burst signal of the input video signal, the system does not disclose a low-pass filter for preventing aliasing. Further, when a mono signal without a burst signal is input, aliasing due to clock variations occurs.
In a camcorder, an external synchronizing circuit for generating a color burst signal when a monochrome video signal or color signal is discriminated according to the presence or absence of a burst signal and a monochrome video synchronizing signal without the burst signal is externally input, is disclosed in Japanese Laid-open Patent Application No. sho 62-219877.
When employed in a camera, the circuit is used to stably reproduce an input color signal by generating a color burst signal using a fixed oscillator, even if the monochrome synchronizing signal without the color burst signal is input.